arXiv subjects

Marta Sewiło

Publications and source records attributed to Marta Sewiło.

30 records · Page 2Linked to original sources

arXiv2019

An ALMA view of molecular filaments in the Large Magellanic Cloud I: The formation of high-mass stars and pillars in the N159E-Papillon Nebula triggered by a cloud-cloud collision

We present the ALMA observations of CO isotopes and 1.3 mm continuum emission toward the N159E-Papillon Nebula in the Large Magellanic Cloud (LMC). The spatial resolution is 0"25-0"28 (0.06-0.07 pc), which is a factor of 3 higher than the previous ALMA observations in this region. The high resolution allowed us to resolve highly filamentary CO distributions with typical widths of $\sim$0.1 pc (full width half maximum) and line masses of a few 100 $M_{\odot}$ pc$^{-1}$. The filaments (more than ten in number) show an outstanding hub-filament structure emanating from the nebular center toward the north. We identified for the first time two massive protostellar outflows of $\sim$10$^4$ yr dynamical age along one of the most massive filaments. The observations also revealed several pillar-like CO features around the Nebula. The H II region and the pillars have a complementary spatial distribution and the column density of the pillars is an order of magnitude higher than that of the pillars in the Eagle nebula (M16) in the Galaxy, suggesting an early stage of pillar formation with an age younger than $\sim$10$^5$ yr. We suggest that a cloud-cloud collision triggered the formation of the filaments and protostar within the last $\sim$2 Myr. It is possible that the collision is more recent, as part of the kpc-scale H I flows come from the tidal interaction resulting from the close encounter between the LMC and SMC $\sim$200 Myr ago as suggested for R136 by Fukui et al.

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arXiv2019

Complex Organic Molecules in Star-Forming Regions of the Magellanic Clouds

The Large and Small Magellanic Clouds (LMC and SMC), gas-rich dwarf companions of the Milky Way, are the nearest laboratories for detailed studies on the formation and survival of complex organic molecules (COMs) under metal poor conditions. To date, only methanol, methyl formate, and dimethyl ether have been detected in these galaxies - all three toward two hot cores in the N113 star-forming region in the LMC, the only extragalactic sources exhibiting complex hot core chemistry. We describe a small and diverse sample of the LMC and SMC sources associated with COMs or hot core chemistry, and compare the observations to theoretical model predictions. Theoretical models accounting for the physical conditions and metallicity of hot molecular cores in the Magellanic Clouds have been able to broadly account for the existing observations, but fail to reproduce the dimethyl ether abundance by more than an order of magnitude. We discuss future prospects for research in the field of complex chemistry in the low-metallicity environment. The detection of COMs in the Magellanic Clouds has important implications for astrobiology. The metallicity of the Magellanic Clouds is similar to galaxies in the earlier epochs of the Universe, thus the presence of COMs in the LMC and SMC indicates that a similar prebiotic chemistry leading to the emergence of life, as it happened on Earth, is possible in low-metallicity systems in the earlier Universe.

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arXiv2019

Relations Between Molecular Cloud Structure Sizes and Line Widths in the Large Magellanic Cloud

We present a comparative study of the size-line width relation for substructures within six molecular clouds in the Large Magellanic Cloud (LMC) mapped with the Atacama Large Millimeter/submillimeter Array (ALMA). Our sample extends our previous study, which compared a Planck detected cold cloud in the outskirts of the LMC with the 30 Doradus molecular cloud and found the typical line width for 1 pc radius structures to be 5 times larger in 30 Doradus. By observing clouds with intermediate levels of star formation activity, we find evidence that line width at a given size increases with increasing local and cloud-scale 8$μ$m intensity. At the same time, line width at a given size appears to independently correlate with measures of mass surface density. Our results suggest that both virial-like motions due to gravity and local energy injection by star formation feedback play important roles in determining intracloud dynamics.

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arXiv2019

Time-Domain Photometry of Protostars at Far-Infrared and Submillimeter Wavelengths

The majority of the ultimate main-sequence mass of a star is assembled in the protostellar phase, where a forming star is embedded in an infalling envelope and encircled by a protoplanetary disk. Studying mass accretion in protostars is thus a key to understanding how stars gain their mass and ultimately how their disks and planets form and evolve. At this early stage, the dense envelope reprocesses most of the luminosity generated by accretion to far-infrared and submillimeter wavelengths. Time-domain photometry at these wavelengths is needed to probe the physics of accretion onto protostars, but variability studies have so far been limited, in large part because of the difficulty in accessing these wavelengths from the ground. We discuss the scientific progress that would be enabled with far-infrared and submillimeter programs to probe protostellar variability in the nearest kiloparsec.

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arXiv2019

Formation of High-Mass stars in an isolated environment in the Large Magellanic Cloud

The aim of this study is to characterize the distribution and basic properties of the natal gas associated with high-mass young stellar objects (YSOs) in isolated environments in the Large Magellanic Cloud (LMC). High-mass stars usually form in Giant Molecular Clouds (GMCs) as part of a young stellar cluster, but some OB stars are observed far from GMCs. By examining the spatial coincidence between the high-mass YSOs and 12CO (J = 1-0) emission detected by NANTEN and Mopra observations, we selected ten high-mass YSOs that are located away from any of the NANTEN clouds but are detected by the Mopra pointed observations. The ALMA observations revealed that a compact molecular cloud whose mass is a few thousand solar masses or smaller is associated with the high-mass YSOs, which indicates that these compact clouds are the sites of high-mass star formation. The high-density and high-temperature throughout the clouds are explained by the severe photodissociation of CO due to the lower metallicity than in the Galaxy. The star formation efficiency ranges from several to as high as ~ 40%, indicating efficient star formation in these environments. The enhanced turbulence may be a cause of the efficient star formation therein, as judged from the gas velocity information and the association with the lower density gas.

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arXiv2018

Identifying Young Stellar Objects in the Outer Galaxy: l = 224 deg Region in Canis Major

We study a very young star-forming region in the outer Galaxy that is the most concentrated source of outflows in the Spitzer Space Telescope GLIMPSE360 survey. This region, dubbed CMa-l224, is located in the Canis Major OB1 association. CMa-l224 is relatively faint in the mid-infrared, but it shines brightly at the far-infrared wavelengths as revealed by the Herschel Space Observatory data from the Hi-GAL survey. Using the 3.6 and 4.5 $μ$m data from the Spitzer/GLIMPSE360 survey, combined with the JHK$_s$ 2MASS and the 70-500 $μ$m Herschel/Hi-GAL data, we develop a young stellar object (YSO) selection criteria based on color-color cuts and fitting of the YSO candidates' spectral energy distributions with YSO 2D radiative transfer models. We identify 293 YSO candidates and estimate physical parameters for 210 sources well-fit with YSO models. We select an additional 47 sources with GLIMPSE360-only photometry as `possible YSO candidates'. The vast majority of these sources are associated with high H$_2$ column density regions and are good targets for follow-up studies. The distribution of YSO candidates at different evolutionary stages with respect to Herschel filaments supports the idea that stars are formed in the filaments and become more dispersed with time. Both the supernova-induced and spontaneous star formation scenarios are plausible in the environmental context of CMa-l224. However, our results indicate that a spontaneous gravitational collapse of filaments is a more likely scenario. The methods developed for CMa-l224 can be used for larger regions in the Galactic plane where the same set of photometry is available.

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arXiv2017

ALMA Observations of a Quiescent Molecular Cloud in the Large Magellanic Cloud

We present high-resolution (sub-parsec) observations of a giant molecular cloud in the nearest star-forming galaxy, the Large Magellanic Cloud. ALMA Band 6 observations trace the bulk of the molecular gas in $^{12}$CO(2-1) and high column density regions in $^{13}$CO(2-1). Our target is a quiescent cloud (PGCC G282.98-32.40, which we refer to as the "Planck cold cloud" or PCC) in the southern outskirts of the galaxy where star-formation activity is very low and largely confined to one location. We decompose the cloud into structures using a dendrogram and apply an identical analysis to matched-resolution cubes of the 30 Doradus molecular cloud (located near intense star formation) for comparison. Structures in the PCC exhibit roughly 10 times lower surface density and 5 times lower velocity dispersion than comparably sized structures in 30 Dor, underscoring the non-universality of molecular cloud properties. In both clouds, structures with relatively higher surface density lie closer to simple virial equilibrium, whereas lower surface density structures tend to exhibit super-virial line widths. In the PCC, relatively high line widths are found in the vicinity of an infrared source whose properties are consistent with a luminous young stellar object. More generally, we find that the smallest resolved structures ("leaves") of the dendrogram span close to the full range of line widths observed across all scales. As a result, while the bulk of the kinetic energy is found on the largest scales, the small-scale energetics tend to be dominated by only a few structures, leading to substantial scatter in observed size-linewidth relationships.

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arXiv2017

ALMA observations of N83C in the early stage of star formation in the Small Magellanic Cloud

We have performed Atacama Large Millimeter/submillimeter Array (ALMA) observations in $^{12}$CO($J=2-1$), $^{13}$CO($J=2-1$), C$^{18}$O($J=2-1$), $^{12}$CO($J=3-2$), $^{13}$CO($J=3-2$), and CS($J=7-6$) lines toward the active star-forming region N83C in the Small Magellanic Cloud (SMC), whose metallicity is $\sim$ 1/5 of the Milky Way (MW). The ALMA observations first reveal sub-pc scale molecular structures in $^{12}$CO($J=2-1$) and $^{13}$CO($J=2-1$) emission. We found strong CO peaks associated with young stellar objects (YSOs) identified by the $Spitzer$ Space Telescope, and also found that overall molecular gas is distributed along the edge of the neighboring HII region. We derived a gas density of $\sim 10^4$ cm$^{-3}$ in molecular clouds associated with YSOs based on the virial mass estimated from $^{12}$CO($J=2-1$) emission. This high gas density is presumably due to the effect of the HII region under the low-metallicity (accordingly small-dust content) environment in the SMC; far-UV radiation from the HII region can easily penetrate and photo-dissociate the outer layer of $^{12}$CO molecules in the molecular clouds, and thus only the innermost parts of the molecular clouds are observed even in $^{12}$CO emission. We obtained the CO-to-H$_2$ conversion factor $X_{\rm CO}$ of $7.5 \times 10^{20}$ cm$^{-2}$ (K km s$^{-1}$)$^{-1}$ in N83C based on virial masses and CO luminosities, which is four times larger than that in the MW, 2 $\times 10^{20}$ cm$^{-2}$ (K km s$^{-1}$)$^{-1}$. We also discuss the difference in the nature between two high-mass YSOs, each of which is associated with a molecular clump with a mass of about a few $\times 10^3 M_{\odot}$.

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arXiv2017

The SAGE-Spec Spitzer Legacy program: The life-cycle of dust and gas in the Large Magellanic Cloud. Point source classification III

The Infrared Spectrograph (IRS) on the {\em Spitzer Space Telescope} observed nearly 800 point sources in the Large Magellanic Cloud (LMC), taking over 1,000 spectra. 197 of these targets were observed as part of the Sage-Spec Spitzer Legacy program; the remainder are from a variety of different calibration, guaranteed time and open time projects. We classify these point sources into types according to their infrared spectral features, continuum and spectral energy distribution shape, bolometric luminosity, cluster membership, and variability information, using a decision-tree classification method. We then refine the classification using supplementary information from the astrophysical literature. We find that our IRS sample is comprised substantially of YSO and H\,{\sc ii} regions, post-Main Sequence low-mass stars: (post-)AGB stars and planetary nebulae and massive stars including several rare evolutionary types. Two supernova remnants, a nova and several background galaxies were also observed. We use these classifications to improve our understanding of the stellar populations in the Large Magellanic Cloud, study the composition and characteristics of dust species in a variety of LMC objects, and to verify the photometric classification methods used by mid-IR surveys. We discover that some widely-used catalogues of objects contain considerable contamination and others are missing sources in our sample.

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arXiv2016

AGB and post-AGB objects in the outer Galaxy

We present the results of our search for low- and intermediate mass evolved stars in the outer Galaxy using AllWISE catalogue photometry. We show that the [3.4]-[12] versus [4.6]-[22] colour-colour diagram is most suitable for separating C-rich/O-rich AGB and post- AGB star candidates. We are able to select 2,510 AGB and 24,821 post-AGB star candidates. However, the latter are severely mixed with the known young stellar objects in this diagram.

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arXiv2016

Stellar evolution in the outer Galaxy

We investigate the distribution of different classes of spectroscopically identified sources and theoretical models in the color-color diagrams (CCDs) combining the near-infrared (NIR) and mid-infrared (MIR) data to develop a method to classify Outer Galaxy sources detected with the Spitzer Space Telescope (hereafter Spitzer) SMOG survey in the IRAC 3.6 and 8.0 micrometer and MIPS 24 micrometer bands. We supplement the Spitzer data with the data from other satellite and ground-based surveys. The main goal of our study is to discover and characterize the population of intermediate- and low-mass young stellar objects (YSOs) in the Outer Galaxy and use it to study star formation in a significantly different environment than the Galaxy inside the solar circle. Since the YSOs can be confused with evolved stars in the MIR, these classes of objects need to be carefully separated. Here we present the initial results of our analysis using the Ks-[8.0] vs. Ks-[24] CCD as an example. The evolved stars separated from YSOs in the YSO selection process will be investigated in detail in the follow-up study.

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arXiv2014

Spitzer View of Massive Star Formation in the Tidally Stripped Magellanic Bridge

The Magellanic Bridge is the nearest low-metallicity, tidally stripped environment, offering a unique high-resolution view of physical conditions in merging and forming galaxies. In this paper we present analysis of candidate massive young stellar objects (YSOs), i.e., {\it in situ, current} massive star formation (MSF) in the Bridge using {\it Spitzer} mid-IR and complementary optical and near-IR photometry. While we definitely find YSOs in the Bridge, the most massive are $\sim10 M_\odot$, $\ll45 M_\odot$ found in the Large Magellanic Cloud (LMC). The intensity of MSF in the Bridge also appears decreasing, as the most massive YSOs are less massive than those formed in the past. To investigate environmental effects on MSF, we have compared properties of massive YSOs in the Bridge to those in the LMC. First, YSOs in the Bridge are apparently less embedded than in the LMC: 81% of Bridge YSOs show optical counterparts, compared to only 56% of LMC sources with the same range of mass, circumstellar dust mass, and line-of-sight extinction. Circumstellar envelopes are evidently more porous or clumpy in the Bridge's low-metallicity environment. Second, we have used whole samples of YSOs in the LMC and the Bridge to estimate the probability of finding YSOs at a given \hi\ column density, N(HI). We found that the LMC has $\sim3\times$ higher probability than the Bridge for N(HI) $>10\times10^{20}$ cm$^{-2}$, but the trend reverses at lower N(HI). Investigating whether this lower efficiency relative to HI is due to less efficient molecular cloud formation, or less efficient cloud collapse, or both, will require sensitive molecular gas observations.

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